How to send data from JavaScript to Drupal 8 form

I am building a form in Drupal 8 that contains a sortable jQuery accordion.

I have the accordion working and am able to add new elements to the accordion utilizing a submit button with an AJAX callback. However, when I reorder the elements in the accordion and then add a new element, the order of the accordion reverts to the original. I have a hidden field for each element in the accordion to store the order in and would like to rebuild the form based on these values, however I am at a loss for how to send this back to Drupal from the Javascript.

I have reviewed some examples like here: Send data from JavaScript to Controller for how to use AJAX to send back to Drupal, however every example I see involves sending back to a menu item, not a form. This is the missing link for me, any help with how I can complete the linkage back to the form is much appreciated.

Here is the routing yml file:

mymodule:
  path: /mymodule
  defaults:
    _form: '\Drupal\mymodule\Form\ContributeForm'
  requirements:
    _permission: 'access content'

Here is a portion of the Drupal form code, including all of it is overwhelming:

$dataArray = array();           
//  Loop through existing fields             
for ($i = 0; $i < $name_field_init; $i++) {
    for($j = 0; $j < $name_field_init; $j++){   

        if($i == $form_state->getCompleteForm()['wrapper']['accordion']["container$j"]["sort_order$j"]['#value']){

            foreach ($form_state->getValues()['wrapper']['accordion']["container$j"] as $key => $value) {
                $dataArray[$i][$key] = $value;
            }
        }//if
    }//for j
}//for i

$i = 0;
foreach($dataArray AS $row){

    $dataVal = "test$i";

    $form['wrapper']['accordion']["container$i"] = array(
      '#type' => 'container',
      '#id' => "container_$i",
      '#name' => $i,
      '#attributes' => array(
        'class' => array(
          'container_l_01',
        ),
      ),
    );

    $form['wrapper']['accordion']["container$i"]["sort_order$i"] = array(
    '#type' => 'hidden',
    '#value' => $i,
    '#attributes' => array(
      'class' => array(
        'sort-order',
      ),
    ),
    );  

    $form['wrapper']['accordion']["container$i"]["title$i"] = array(
      '#markup' => "<h3>$dataVal</h3>",
    );

    $form['wrapper']['accordion']["container$i"]["textbox$i"] = array(
      '#type' => 'textfield',
      '#title' => "$dataVal field",
      '#default_value' => $row["textbox$i"],
    );

    $i++;

}//foreach

Here is the javascript:

(function ($) {

  'use strict';

  Drupal.behaviors.mymodule = {
    attach: function(context, settings) {
      $(document).ready(function () {

        $('#accordion').accordion({
          collapsible: true,
          active: false,
          height: 'fill',
          header: 'h3'
        }).sortable({
          items: '.container_l_01',
          stop: function(event, ui) {
            var data = "";

            $(".container_l_01").each(function(i, el){
                data += $(el).attr('id') + '=' + $(el).children('.sort-order').val() + ',';            
            });

            alert(data);
          }
        });

        $('#accordion').on('accordionactivate', function (event, ui) {
          if (ui.newPanel.length) {
            $('#accordion').sortable('disable');            
          } 
          else {
            //alert("entered the else");
            $('#accordion').sortable('enable');            
          }
        }); 

      });
    }
  }
})(jQuery);